High intensity far u.v. radiation source

ABSTRACT

A far U.V. emitting vapor electric discharge lamp which emits selectively in the wavelength band of approximately 2,000 to 2,300 A.U. utilizes a light emitting media of a vaporizable ionizable metal such as cadmium or zinc at very low pressure. High current density in excess of 1 ampere per square centimeter is used to excite metal to high intensity emission in an ambient of a noble gas at low pressure. In accord with a preferred embodiment, cadmium or zinc vapors are excited in the presence of xenon gas. The xenon has metastable states which enter into the metal vapor excitation mechanism, resulting in increased intensity of emission at the wavelengths characteristic of the metal species under these conditions of current density and pressure.

United States Patent [151 3,657,590 [4 1 Apr. 18, 1972 Johnson [54] HIGH INTENSITY FAR U.V.

RADIATION SOURCE [72] Inventor: Peter D. Johnson, Schenectady, NY. [73] Assignee: General Electric Company [22] Filed: June 26, 1970 [211 Appl. No.: 50,105

[52] U.S. Cl ..313/223, 313/27, 313/227 [51] Int. Cl. ..H0lj 17/20 [58] Field oiSearch ..313/25, 27,47,223

[56] References Cited UNITED STATES PATENTS 2,194,300 3/1940 Found ..313/27 X 2,228,327 l/l94l Spanner ..313/2'7X Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman 57 ABSTRACT A far U.V. emitting vapor electric discharge lamp which emits selectively in the wavelength band of approximately 2,000 to 2,300 A.U. utilizes a light emitting media of a vaporizable ionizable metal such as cadmium or zinc at very low pressure. High current density in excess of 1 ampere per squarecentimeter is used to excite metal to high intensity emission in an ambient of a noble gas at low pressure. In accord with a preferred embodiment, cadmium or zinc vapors are excited in the presence of xenon gas. The xenon has metastable states which enter into the metal vapor excitation mechanism, resulting in increased intensity of emission at the wavelengths characteristic of the metal species under these conditions of current density and pressure.

10 Claims, 3 Drawing Figures HIGH INTENSITY FAR U.V. RADIATION SOURCE The present invention relates to far ultraviolet light sources. More particularly, the invention relates to such sources which emit radiation of wavelengths having enhanced photochemical stimulation properties otherwise previously unobtainable at useful intensities and efficiencies. This application is related to my co-pending, concurrently filed applications, Ser. No. 50,106 and Ser. No. 50,203.

Electric lamps emitting ultraviolet radiation generally utilize a gaseous discharge utilizing mercury as the working gas. In most prior art devices utilized for this purpose, the lamp parameter of low current (below 0.2 amperes per square centimeter) and low pressure of emitting specie (below I torr), are such that the principal radiation supplied is above 2,300 A.U., primarily that of the 2,537 A.U. line which is so strong, under the parameters of the prior arts as generally to dominate such ultraviolet emission. l have found that, although the 2,537 A.U. ultraviolet wavelength emission is useful for many purposes, it is very inefficient in causing many photochemical reactions, as for example, crosslinking of polymers and breaking of polymeric bonds. Other U.V. emitting lamps, which emit shorter wavelength radiation, are operated at high pressure (several atmospheres) and high current, (above 1 ampere per square centimeter), but still only emit useful U.V. radiation at wavelengths longer than 2,300 A.U., which radiation is not effective for many photochemical reactions, particularly the crosslinking of many polymers.

In my co-pending, concurrently filed application, Ser. No. 50,106, I have disclosed and claimed broadly new far U.V. emitting lamps which include the concept of operating an ionizable metal vapor electric lamp at very low pressures (less than 1 torr) and very high currents (greater than 0.5 amperes/cmF), resulting in the emission of high intensity far U.V. radiation at wavelength of less than 2,300 A.U. Thus, for example, mercury vapor at a pressure of less than 0.75 a/cmFwith currents of 0.5 to 25 amperes/cm. emits high intensity radiation at l,849 and 1,942 A.U. in an ambient of helium, argon or neon.

Despite the advantages of such lamps it is desirable to provide further and different sources of short wavelength U.V. radiation in order to make photochemical processes as efficient and selective as to wavelength as is possible.

Accordingly, it is an object of the present invention to provide metallic vapor-electric discharge lamps having different effective wavelengths of far violet emission at high intensity.

Still another object of the inventionis to provide metallic vapor-electric lamps with emission peaks at new and different far U.V. wavelengths.

Briefly stated in accord with one embodiment of the present invention, l provide an evacuable envelope containing a quantity of cadmium or zinc sufficient to yield, under operating minimum bulb wall temperatures of 200 to 450 C, an operating pressure metal vapor of approximately to 1.0 torr, permitting the emission of a high intensity of far ultraviolet radiation, largely at wavelengths of 2,000 A.U. to 2,300 A.U.

The novel feature believed characteristic of the present inr vention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the following detailed description, taken in connection with the appended drawing in which FIG. 1 is a horizontal view, with parts broken away, of a lamp constructed in accord with the present invention and suitable for operation for the production of high intensity photochemically-useful far ultraviolet light, and

FIG. 2 is an alternative embodiment to the device of FIG. 1, and

FIG. 3 is a graph containing typical curves for respective wavelength emissions illustrating intensity as a function of lamp current density of typical lamps constructed in accord with the present invention.

FIG. 1 illustrates a simplified far ultraviolet lamp in accord with the present invention. The lamp of FIG. 1 includes an evacuable U.V. transmissive outer envelope represented generally at l and containing a second inner, hermetically sealed'envelope 2, which includes an ultraviolet transmissive central member 3 and a pair of enlarged electrode-containing end members 4 and 5, respectively. Each of end members 4 and 5 includes a cathode or filament 6 which, in this instance, is independently heated and, is mounted between the inboard ends of a pair of inleads 7 and 8, respectively. Each of inleads 7 and 8 has connected, immediately adjacent the filament and substantially parallel thereto, a pair of auxiliary anode members 9 and 10, respectively. These members, on alternate cycles of an alternating current voltage exciting the device under alternating current operation, serve as anode members to sustain an electric discharge.

Inner envelope or discharge bulb 2 is supported at the upper and lower ends by a pair of saddle clamps l1 and 12, which are supported by support leads l3 imbedded envelope in outer envelope nipples 14. Alternatively, a single ended arrange ment, as is generally utilized to support innner envelopes in standard high pressure mercury lamps with a grid structure may be used. Heat shields 17 and I8, similarly supported, surround each of end members 4 and 5 to conserve heat and maintain the temperature of the end members as hot as possible. The region between outer envelope and inner envelope 2 is evacuated to facilitate the maintenance of the necessary high operating temperatures within envelope 2. g

The desired current and voltage to operate the lamp are supplied by a power supply means, capable of supplying the requirements of the lamp in operation and may take a number of forms, but which is, for example, illustrated generally as a saturable transformer represented by dotted line box 19. Transformer 19 includes a primary winding 20 and a voltage step-up secondary winding 21 having a pair of tapped secondary low voltage portions 22 and 23 which are connected to terminals 15-16 across respective pairs of electrode inleads 7 and 8 on either end of the lamp so as to provide alternating current heating of each of the filaments 6 by the voltage developed across the tapped secondary. This voltage is neces sarily low, of the order of several volts, to cause heating of the filament to sustain the thermionic emission of electrons to sustain an electric discharge between the filament and anode members are respective opposite ends of the arc tube. The filaments are essentially similar to conventional fluorescent lamp filaments, although designs permitting high currents of the order of 25 amperes/cm. are preferred. The lamp is charged with a low pressure of a noble gas and a charge 24 of a sufficient quantity of a vaporizable ionizable metal selected from the group consisting of cadmium and zinc.

In operation, the lamp is started by the application of a line voltage, which may be of any desired voltage but which may conveniently be or 240 volts, to the primary of the excitation of the filaments 6, the lamp is immediately operative. A quantity of a vaporizable, ionizable metal, either cadmium or zinc, is present as charge 24 within the lamp, as is a filling of a relatively low pressure, as for example, 1.0-25 and preferably approximately 2-5 torr of an inert noble gas such as, helium, argon, or neon. The application of the initial voltage causes an electric discharge to be sustained by the noble gas, which is immediately ionized thereby, which ionized gas discharge maintains the bulb wall at a temperature consistent with the desired vapor pressure of the metallic specie within the lamp envelope, to cause the establishment of a sufficient pressure of cadmium or zinc to permit ionization thereof and transfer of the discharge to the metal vapor as the conducting specie. Due to the thermal isolation of the inner envelope 2 within envelope 1 by a suitable vacuum or a low pressure fill of a low thermal conductance gas such as nitrogen, as is well known to the art, makes the attainment and maintenance of high temperatures for the inner envelope innerwall of the order of 200 to 450 C readily feasible. If on the other hand, due to a desire to keep the possible source of dimunition of the U.V. emission to a minimum, it is desired to have only one envelope, then suitable heating means, as for example, auxiliary heater coils or resistances may be juxtaposed within or about the sole envelope wall. Such heating means are then readily controlled to maintain the envelope inner \\tlll temperature within the desired operating range of approximately 200-450 C.

FIG. 2 illustrates another embodiment of the invention, functionally equivalent to the device of FIG. I, but having certain structural modifications, illustrating the versatility with which structure embodying the invention may be constructed. In FIG. 2, like parts to those in the device of FIG. I bear like numerals.

In FIG. 2, an outer evacuable U.V. transmissive envelope 1 includes an inner envelope 2, similarily supported and having at least a portion thereof ultraviolet transmissive.

End portions 4 and 5 of the inner envelope of the device of FIG. 2, adapted to be operated with direct current excitation, respectively include a cathode assembly 25 and an anode assembly 26. Cathode assembly 25 may conveniently have the so-called Ml cathode structure, that is, a dispenser type filament member 27 containing a single loop and fabricated from a mesh stocking container containing a particulate mass of a thermionic emitting substance, such as barium aluminate or lanthanum boride, for example. A cathode shield member 28 laterally surrounds the filament member 27 and contains an aperture 29 therein for the escape of electrons to sustain an electric discharge along the axis of inner envelope 2. Anode assembly 26 in its simplest structure, contains a collector means which in this instance, is shown as a hollow cup 30 mounted upon an inlead 31 which passes through pinch 32 in the inner envelope end, just as filament 27 is mounted upon, and electrically connected between, inleads 33 and 34, which pass through pinch 35 in the opposite end of the inner envelope. It is desirable that cathodes and anodes of these general types, respectively, be used under direct current excitation in order to maintain the high rate of current conduction in the steady state as may be required, although it is to be appreciated that for direct current excitation, any suitable cathode and anode structure which are able to maintain current densities of up to 25 amperes per square centimeter in the discharge but preferably up to amperes per square centimeter are suitable.

Similarly, although a particular type of electrode structure is shown in the device of FIG. 1 for the maintenance of the high current density characteristic of lamps in accord with the AC. excited embodiment of the present invention, it should be appreciated that any similar electrode structure which is capable of maintaining the aforementioned current densities under alternating current excitation is suitable. Alternatively, for alternating current operation, a lamp may be fabricated without electrodes and excitation thereof may be accomplished in accord with the technique described in application Ser. No. 653,749, filed July 17, 1967, in the name of]. M. Anderson, and assigned to the present assignee, now US. Pat. No. 3,500,118. In such an arrangement, a closed loop constitutes the discharge path within an hermetically sealed envelope containing an appropriate ionizable fill. A radiofrequency oscillator is connected to a primary winding which is coupled to the closed loop through a high radio-frequency permeable ferrite core. The secondary of the excitation transformer is the discharge path through the closed loop of the lamp which passes through a portion of the core and is excited thereby. Such systems operate ideally at frequencies of 500 kilocycles and above and are suitable for the production of high current density at low pressure in accord with the present invention, wholly independent of electrode phenomena.

In the embodiment of FIGS. 1 and 2, a fill of a noble gas such as helium, neon or argon of a pressure of approximately 1.0-25 torr and preferably approximately 2-5 torr is contained within the envelopes. Similarly, a suitable quantity of zinc, or cadmium is included within the envelope and is adaptable, upon heating by an initial discharge established within the noble gas, to be vaporized and ionized such as to cause the discharge essentially to become a metallic vapor discharge at low pressure an high current density for efficient emission of wavelength radiation in the far ultraviolet between approximately 2,000 A.U. and 2,300 A.U.

The law s nttlw present invention are parliament useful in producing igh intensity at high efficiencies of tar U.V. radiation within the range of approximately 2,000 A.U. to 2,300 A.U. The conventional mercury lamps used for U.V. sources in the prior art do not effectively emit below 2,300 A.U. The lamps of my aforementioned co-pending applications emit principally at wavelengths below 2,000 A.U., i.e., at principal wavelengths of 1,849 A.U. and 1,942 A.U. The lamps of the present invention, then, help to make available U.V. radiation in a spectral region not attainable in the prior art or in my copending application. It should be appreciated that in photochemical reactions, the sensitivity of a given reaction to fat U.V. radiation, is usually very sensitive to wavelength and lamps of the present invention make possible photochemical reactions not heretofore attainable.

Lamps containing cadmium as the vaporizable ionizable emitting specie in accord with the present invention emit strongly at the atomic resonance radiation wavelength of 2,288 A.U. and at the ionic emission wavelength of 2,144 and 2,265 A.U. Thus, cadmium emitting lamps are useful to fill out the narrow band of approximately 2,140 to 2,290 A.U. with high intensity U.V. radiation. Lamps containing zinc as the radiating specie, emit strongly at the atomic resonance wavelength of 2,139 A.U. and at the ionic emission wavelengths of 2,026 and 2,062 A.U., thus providing a source of high intensity far U.V. radiation at the wavelength band from approximately 2,020 to 2,140 A.U., complementing the cadmium wavelength band.

In lamps containing cadmium or zinc, I am able to secure useful high intensity far U.V. radiation with the inner bulb or envelope containing a partial pressure of radiating specie of from approximately 10 to 1.0 torr in equilibrium with some unvaporized metallic specie. Preferably, however, for optimum emission efficiency, I prefer to operate with a partial pressure of approximately 3X10 to 3X10 torr of the emitting specie. I

The foregoing operating partial pressures of radiating species are obtained when zinc is the radiating specie, by maintaining the bulb wall minimum temperature at a value of approximately 250 to 450 C, but to obtain the optimum partial pressure, at a temperature approximately 300 to 400 C. When cadmium is the emitting specie, the operative range of pressures are obtained when the temperature of the coolest portion of the lamp inner envelope is maintained at a temperature of approximately 200 to 400 C, although to obtain the optimum operating pressures, I prefer to operate the envelope wall at a temperature of approximately 250 to 350 C. In all instances, the inert gas pressure may be any value from approximately l.0 to 25 torr, although Iprefer to utilize approximately 2-5 torr for optimum efficiency.

The low pressure of radiating specie within the lamps in accord with the present invention is maintained primarily by maintaining control of the coldest portion of the .lamp envelope wall, generally in the region of the outboard end of the lamp near cathode and anode electrodes. As hereinafter utilized, the terms bulb wall temperature" or envelope wall temperature will be utilized to identify the minimum temperature at which any portion of the interior of the inner bulb or envelope wall exists during steady state operation. This temperature is utilized, as a frame of reference, because it effectively controls the pressure of the metal vapor within the lamp, by virtue of the fact that the pressure of metal vapor within the lamp is controlled by the degree of condensation of the vapor at the coldest portion of the bulb wall.

The current density within the wall is maintained at the desired range by appropriately adjusting the total current through the discharge and the diameter of the narrow, U.V. transmissive portion of the lamp envelope. The totalcurrent is controlled by external impedances, and is adjusted to obtain maximum output from the radiating spectroscopic states of the radiant species.

In general, for alternating current operation, lamps in accord with the present invention may readily operate at a voltage from 20 to 100 volts A.C. at a current density of 0.5 to amperes per square centimeter, although operation at current densities up to 25 amperes per square centimeter is quite useful. A typical lamp configuration for the attainment of such operation, namely, at a current density of approximately 10 amperes per square centimeter and a pressure of approximately 3 torr, may readily be obtained within a lamp envelope having an inner interior bulb diameter within the ultraviolet transmissive region of approximately 1 centimeter and an interelectrode length of approximately 25 centimeters with an applied voltage of 25 volts.

FIG. 3 of the drawing illustrates typical plots of lamp intensity, in arbitrary units, plotted as a function of lamp current density in amperes per square centimeter forthe output of the 2,265 A.U. cadmium line (Curve A) and of the total emission of a lamp containing zinc as the radiating specie (Curve B).

Different arbitrary unit scales were used for each curve. As

may be seen from the plots of FIG. 3, both emitting specie exhibit an exponential slope and indicate a rapidly increasing emission with current density, providing excellent emission at high efficiencies, particularly at current densities in excess of approximately 4 a/cmF. As a practical matter, for most modes of operation, a useful maximum of approximately 25 amperes per square centimeter is obtainable. It is to be noted, however, that high density alumina or high density yttria or other similar ultraviolet transmissive ceramic envelopes may be utilized at the desired temperature of operation. Accordingly, it is within the scope of the present invention that other U.V. transmissive materials than quartz extending into the high density translucent and ultraviolet transmissive ceramics may be utilized in accord with the present invention. Lamps operated in accord with the present invention have very high intensities of emission in the 2,000 A.U. to 2,300 A.U. range. These wavelengths, heretofore emitted only trivially by lamps of the prior art, are emitted with high efficiencies.

I have found that the scientific characterization of the lamp in accord with the invention has been that of a high electron temperature within the positive column of the discharge and that the discharge is distributed over the entire positive column and is not a mere cathode or anode phenomenon. This is particularly evidenced by the fact that the lamps in accord with the present invention may be operated in electrodeless environments. With this high electron temperature, I find that a high percentage of the excited cadmium or zinc specie exists in the atomic P" spectroscopic state and the ionic 1, and P spectroscopic states.

In accord with another feature of the present invention, I provide a sufficient quantity of radiating specie to the inner lamp envelope so that there is always an excess thereof within the lamp and in the vicinity of the coldest portion of the bulb wall. This provides for cleaning up of the emitting specie by deposition of metal upon lamp parts or on the lamp envelope wall without decreasing the pressure below the desired predetermined pressure at which the lamp is to operate. As is mentioned hereinbefore, the lamps of the present invention are governed according to the interior diameter of the bulb wall in order that the current density for any given current may be controlled, although this control is not the exclusive factor. In order to maintain the current (and hence the predetermined current density) within the appropriate range, the voltage source is chosen to have an internal limiting impedance (or an external impedance is supplied) so that the current is limited, as for example, by saturation of a saturable transformer. The operating voltage is that required by the discharge path size and shape. Thus, for example, a given discharge of cadmium at 0.01 torr may require a voltage of 1 volt per cm. in argon at 3 torr ambient. A 25 cm. long inner discharge tube of fused quartz having an inner diameter of about 1 cm. within an outer envelope of fused quartz 40 cm. long having an ID of 5 cm. requires a voltage of approximately 35 volts. Further choices of operating voltages are well withinthe purview of one skilled in the art. Generally, however, the lamps of the invention operate on a voltage which is generally of the order of 20 to volts, with an appropriate impedance, and the interior inner bulb wall diameter varies over a range of approximately 3 to 40 millimeters, but is preferably maintained within the range of 5 to 25 millimeters, for maximum intensity and efficiency of U.V. light output. The length of the ultraviolet transmissive portion of the lamp may be any value above about 10 centimeters with no substantial upward limit, the upward limit being substantially governed by the configuration to which the lamp must conform for operational purposes.

In my aforementioned concurrently-filed co-pending application, Ser. No. 50,203, I have disclosed and claimed an improvement in the lamps disclosed and claimed in my concurrently filed co-pending application, Ser. No. 50,106. In the latter of my Co-pending application, I describe and claim lamps utilizing a high current density, low pressure discharge in mercury vapor to produce far U.V. radiation at wavelengths shorter than 2,000 A.U. in an ambient of low pressure of low molecular weight inert gasses such as helium, neon or argon. In such lamps, the inert gas has a high ionization potential with respect to the ionization potential of mercury so that the inert gas serves primarily as a starter and buffer gas and does not enter into the light emission reactions.

In my other concurrently filed co-pending application, Ser. No. 50,203, I describe and claim lamps utilizing my discovery that lamps, as described above, but utilizing a krypton gas fill, operate in a different mode and that the krypton enters into the light emitting mechanism to cause an unexpected intensification of the emission of U.V. radiation therefrom by a factor of 2 or 3, depending upon current density. This is due to the fact that the krypton possesses metastable excited states approximating the ionization potential of the mercury so that the krypton is adapted to transfer energy to the mercury vapor, greatly increasing its energy state and resulting in higher intensity of emission.

I find that in a preferred embodiment of the present invention, a similar mechanism may be utilized by choosing xenon as the inert gas. Although the use of helium, argon and neon is of great utility in constructing high intensity far U.V. lamps to emit selectively in the 2,000 A.U. to 2,300 A.U., U.S. range, when the cost is not a primary objective, and highest intensity of emission is the objective, the use of xenon gives added and desirable increase in emission intensity.

The operation of lamps in accord with this embodiment of the invention, utilizing a fill of cadmium or zinc and xenon, functions substantially according to the mechanism described theoretically in greater detail in my aforementioned co-pending application, Ser. No. 50,203. Specifically, the excited xenon gas has metastable states at approximately 9.4 eV( P,,) and 9.55 eV(P which are able to transfer energy to cadmium, with an ionization potential of approximately 9.0 eV, and zinc, having an ionization potential of approximately 9.5 eV, to cause a more excited state in the emitting vapor specie and the emission of higher intensities of far U.V. emission at the same wavelengths as emission occurs when lighter noble gases are used.

Lamps, in general, constructed in accord with the present invention are of great utility in stimulating photochemical reactions. More particularly, the photopolymerization of thin monomeric films in the production of photoresist-like substance is one very useful purpose to which lamps in accord with this invention may be placed. In particular, a very sub stantial use has been found in the photopolymerization of hexachlorobutadiene. Photopolymerization of hexachlorobutadiene with ultraviolet radiation is described, from the chemical viewpoint, in greater detail in the application of A. N. Wright, Ser. No. 648,132, filed Feb. 23, 1967. Other uses for lamps of the present invention are in the radiation of other .photoresists and the stimulation of various chemical reactions such as photosynthesis. Previously, commercially available ultraviolet light sources have been utilized for this purpose, but

- 7 all such low pressure, low current lamps emit predominantly the 2,537 A.U. mercury line. Similarly, high pressure, high current mercury vapor lamps have been used to emit shorter wavelength U.V. radiation,- but still at wavelengths of 2,300 A.U. or longer. Such wavelengths are not as effective for many chemical reactions which ultraviolet selectively responsive to effective radiation wavelengths of the other of 2,000 to 2,300 A.U.

By the foregoing, I have described new and improved far ultraviolet emitting vapor discharge lamps selectively emitting ultraviolet radiation, in the 2,000 to 2,300 A.U. range, which has high efficiencies of up to 30 percent and high luminous intensity in the far ultraviolet and which are selectively useful for stimulating photochemical reactions.

While the invention has been described herein with respect to certain specific examples and preferred embodiments thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the sphere and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An ultraviolet lamp emitting selectively at high emission intensity in the region of 2,000 to 2,300 A.U. and comprising:

a. an evacuable discharge envelope having at least a portion thereof transmissive to U.V. radiation of wavelengths of approximately 2,000-2,300 A.U. b. a filling within said envelope including b,'. a partial pressure of a noble gas at a pressure of approximately 1.0 to 25 torr b;. a quantity of a vaporizable ionizable U.V. lightemitting metal selected from the group consisting of cadmium and zinc at an operating partial pressure of approximately 10" to 1.0 torr.

c. means juxtaposed in heat-coupled relation with said envelope to maintain the coolest portionof the interior of said discharge envelope at a temperature of approximately 200 to 450 C. during lamp operation;

d. means coupled with said filling for applying an electric voltage thereto,

d,. said voltage being sufficient to establish an electric discharge within said discharge envelope and cause the creation of an U.V. emitting plasma therein;

e. said plasma sustaining a current density of approximately 1.0 to 25 amperes per square centimeter and emitting far U.V. at wavelength within the range of 2,000 to 2,300 A.U.

2. The lamp of claim 1 wherein said vaporizable ionizable metal is cadmium, said discharge envelope wall temperature is maintained in the operating range of approximately 200 to 400 C and said U.V. emission is within the wavelength band of approximately 2,140 to 2,290 A.U.

3. The lamp of claim 2 wherein said discharge envelope wall is maintained at a temperature of approximately 250 to 350 C and the partial pressure of cadmium within said discharge envelope is approximately 3X10 to 3X 1 0 torr.

4. The lamp of claim 1 wherein said vaporizable ionizable metal is zinc, said discharge envelope wall temperature is maintained at a temperature of approximately 250 to 450 C and said U.V. emission is within the wavelength band of approximately 2,020 to 2,140 A.U.

5. The lamp of claim 4 wherein said discharge envelope wall is maintained at a temperature of approximately 300 to 400 C and the partial pressure of zinc within said discharge envelope is approximately 3X10" to 3X10" torr.

6. The lamp of claim 1 wherein said means for maintaining said discharge envelope wall at said temperature during lamp operation includes an hermetically sealed U.V. transmissive envelope enclosing discharge envelope and containing an interposed low pressure volume having poor thermal conduction characteristic to prevent heat loss from said discharge envelope.

7. The lamp of claim 6 wherein said interposed volume is evacuated.

8. The lamp of claim 7 wherein said interposed volume is filled with a low conductivity gas at reduced pressure.

9. The lamp of claim 1 wherein said noble gas within said envelope is at a pressure of approximately 25 torr.

10. The lamp of claim 1 wherein said noble gas is xenon. 

2. The lamp of claim 1 wherein said vaporizable ionizable metal is cadmium, said discharge envelope wall temperature is maintained in the operating range of approximately 200* to 400* C and said U.V. emission is within the wavelength band of approximately 2,140 to 2,290 A.U.
 3. The lamp of claim 2 wherein said discharge envelope wall is maintained at a temperature of approximately 250* to 350* C and the partial pressure of cadmium within said discharge envelope is approximately 3 X 10 3 to 3 X 10 1 torr.
 4. The lamp of claim 1 wherein said vaporizable ionizable metal is zinc, said discharge envelope wall temperature is maintained at a temperature of approximately 250* to 450* C and said U.V. emission is within the wavelength band of approximately 2,020 to 2,140 A.U.
 5. The lamp of claim 4 wherein said discharge envelope wall is maintained at a temperature of approximately 300* to 400* C and the partial pressure of zinc within said discharge envelope is approximately 3 X 10 3 to 3 X 10 1 torr.
 6. The lamp of claim 1 wherein said means for maintaining said discharge envelope wall at said temperature during lamp operation includes an hermetically sealed U.V. transmissive envelope enclosing discharge envelope and containing an interposed low pressure volume having poor thermal conduction characteristic to prevent heat loss from said discharge envelope.
 7. The lamp of claim 6 wherein said interposed volume is evacuated.
 8. The lamp of claim 7 wherein said interposed volume is filled with a low conductivity gas at reduced pressure.
 9. The lamp of claim 1 wherein said noble gas within said envelope is at a pressure of approximately 2-5 torr.
 10. The lamp of claim 1 wherein said noble gas is xenon. 